Field of the Invention
The invention relates to a gating element for modulating or switching of ion currents in ion mobility spectrometers (IMS) wherein, in particular, the ion current of an ion source is modulated with a continuous modulation function and the mobility spectrum is generated from the measured ion current by a correlation analysis with the modulation pattern.
Description of the Related Art
In most cases, ion mobility spectrometers are operated by injecting very short ion current pulses. A conventional ion mobility spectrometer for the measurement of pollutants, drugs or explosives in air is shown in FIG. 1. The ions are generated continuously in an ion source (2) and then introduced into the drift region of the spectrometer by a gating grid (4) over a short time span. The time spans for the transmission are usually between 100 and 300 microseconds, and the acquisition of the spectrum takes around 30 milliseconds. Bipolar wire grids according to Bradbury-Nielsen are often used as gating grids.
The ions transmitted by the grid (4) are then drawn by an axial electric field through a collision gas in a drift region (8) to the Faraday detector (9). Their velocity is determined by their “mobility”, which in turn depends in a known way on their collision cross-section, their mass, their ability to become polarized, and their tendency to form complex ions with molecules from the collision gas. The drawing field is formed by a series of electrodes (7), which surround the drift region (8) and to which linearly increasing or decreasing potentials are applied, depending on the electric charge of the ions. From the molecules of a substance which enters the ion source (2) together with ambient air (1), several ionic species are formed in the ion source (2), such as monomers, dimers and complex ions with water and collision gas molecules, usually in complex series of ionization reactions, for example by means of radioactive radiation from an emitter (3). Each ionic species has its own characteristic mobility. At the end of the drift region (8), the incident ion current is measured with an ion detector (9), digitized and saved as a “mobility spectrum” in the form of a digitized sequence of measured values. An evaluation of this mobility spectrum provides information on the mobility of the ions involved and hence indications as to the substances involved. (There are also ion sources which operate without radioactivity).
The method is extraordinarily sensitive in respect of certain groups of substances and is largely used for the measurement of pollutants in air, for example for monitoring of chemical laboratories, for continuous monitoring of filters, for control of drying processes, for monitoring of waste air, and for detection of chemical warfare agents, explosives, drugs, and so forth.
For a conventional spectral measurement repetition rate of about 30 spectra per second, and an ion transmission time of between 150 and 300 microseconds, only between one-half and one percent of the ions of a substance which is introduced in gas phase state are actually utilized. The remaining ions are discharged, predominantly at the gating grid (4), and are lost to the measurement process.
The patent specification DE 10 2008 015 000 B4 (U. Renner; GB 2 458 368 B; U.S. Pat. No. 8,304,717 B2) describes a method in which the ion current in the ion mobility spectrometer is modulated at the gating element (4) by a continuous modulation function, and the mobility spectrum is generated from the measured ion current by means of a correlation analysis with the modulation pattern. Preferably, a gating element (4) is used for the modulation which comprises a characteristic that is as linear as possible, since otherwise interfering sidebands occur, which could wrongly be assumed to be real signals. A favorable modulation function is a “chirp”, i.e., a sine function whose frequency is continuously tuned from a lower limit to an upper limit and repeated periodically in a continuous measurement mode.
The patent specification DE 10 2009 025 727 B4 (U. Renner; GB2471745B; U.S. Pat. No. 8,198,584 B2) explains how interfering sidebands in the mobility spectrum, which are generated as a result of the non-linear behavior of the modulating gating element (4), can be reduced by a systematically pre-distorted modulation function. It is nevertheless advantageous to use a gating element (4) with a characteristic which is as straight as possible.
The conventional bipolar, coplanar wire grids according to Bradbury-Nielsen are not advantageous for the analog modulation because the transverse fields between the wires cause a lateral deflection of the migrating ions. It is better to use two (or more) unipolar gating grids in series, whose counter voltages or attracting voltages act essentially in the axial direction. They are known as “Tyndall-Powell gates”.
The published patent application US 2008/0179515 A1 (R. P. Sperline, 2007) describes, in particular, ion-collecting gating methods for ions in ion mobility spectrometers which comprise Tyndall-Powell gating elements with two, three or four free-standing grids and which generate short pulses of ions.
The published patent application WO 2015/194943 A1 (S. V. Mitko, 2014) discloses gating grids (“shutters”) according to Tyndall-Powell with three or four grids, wherein two grids which can be supplied with different potentials are located on the front and rear of an electrode plate with elongated apertures such that two coplanar line grids result. The outermost grids are present to ensure that the field gradients in the spectrometer on both sides of the gating element are disturbed as little as possible. Operation comprises a pulsed switching on and off of the ion current to generate short ion current pulses.
The U.S. Pat. No. 7,417,222 B1 (K. B. Pfeifer and S. B. Rhode, 2005) describes a correlation ion mobility spectrometer. A gating element modulates the ion current, and the mobility spectrum is obtained by means of a correlation analysis of the ion current pattern with the modulation function. A gating grid according to Bradbury-Nielsen as well as a pair of grids according to Tyndall-Powell are proposed as the gating element.
If a mobility spectrometer is to be miniaturized, the use of grids as gating elements is unfavorable. The necessarily very thin wires are susceptible to vibrations, which impair the switching or modulation function. It is, moreover, difficult to shape and arrange the grids such that the modulation curve created is as linear as possible.
The objective is to find a gating element with which the ion current from a continuously operating ion source of an ion mobility spectrometer can be switched or modulated, which is mechanically stable, and simple and low cost to manufacture and operate. The gating element should have a characteristic that is as linear as possible in order to suppress interfering sidebands in the mobility spectrum, especially in the case of continuous modulation of the ion current and subsequent correlation analysis of the measured ion current.